1. Field of the Invention
The present invention relates to the field of mode-switching transformers, generally called baluns (for balance-unbalance). Such transformers are intended to convert a common-mode signal (referenced to ground) into a differential mode signal and conversely, and to avoid disturbances introduced by a ground plane of the circuit, a parasitic coupling, etc.
An example of application of baluns of the type to which the present invention applies relates to baluns equipping portable phones which convert the signals received by the telephone antenna into signals exploitable by its circuits and, in the reverse direction, of enabling transmission of signals by the antenna.
2. Discussion of the Related Art
FIG. 1 very schematically shows the block diagram of a balun 1 and FIG. 2 shows its equivalent electric diagram.
This transformer comprises a common-mode access 2, generally connected to an antenna 3, and two differential mode accesses 4 and 5 intended to be connected to circuits for processing received signals or signals to be transmitted. Electrically, transformer 1 is equivalent to four inductances L1, L2, L3, and L4 (FIG. 2) associated in series two by two, each inductance of a branch (common mode or differential mode) being coupled to an inductance of the other branch. For example, two inductances L1 and L2 are in series between common-mode access 2 and an access 6 generally left in the air. Two inductances L3 and L4 are in series between differential-mode accesses 4 and 5, their junction point being generally connected to ground 7.
Baluns are distributed in three categories. A first category relates to so-called distributed baluns formed of transmission lines coupled to one another. A second category relates to so-called active baluns formed of transistors. A third category relates to so-called baluns with local elements formed of inductive elements and possible discrete capacitive elements. The present invention relates to distributed baluns of the first category.
Distributed baluns are generally formed by means of conductive tracks deposited on a substrate. These tracks have the shape of planar windings forming the inductive elements and have lengths which are a function, especially, of the operating frequency range desired for the balun. The present invention more specifically relates to so-called Marchand-type baluns, the conductive windings of which have lengths which are a function of the quarter of the wavelength corresponding to the central frequency of the passband desired for the balun.
A problem which is posed in the assembly of a balun in the radiofrequency transceiver chain for which it is intended originates from the impedance matching between the different elements.
FIG. 3 shows a conventional example of the assembly of a balun 1 such as described previously in its application environment. Differential-mode accesses 4 and 5 are intended to be connected to input-output terminals of a circuit 8 (IC), generally, an integrated circuit of exploitation and generation of the communication signals. The connection between balun 1 and circuit 8 is generally performed via at least one impedance matching element 9 (Z) and, possibly, a filter (not shown). For simplification, ground 7 has not been shown in FIG. 3.
Impedance matching element 9 has a unity impedance ratio (for example, 50 ohms/50 ohms, 200 ohms/200 ohms, etc.) or not (for example, 50 ohms/100 ohms, 400 ohms/75 ohms, etc.) according to the needs of the application.
The insertion of an impedance matching element between the balun and the circuits exploiting its signals adversely affects the need to miniaturize electronic circuits. It would thus be desirable to be able to combine them.
However, balun sizing aims are a priori incompatible with those of an impedance matcher, especially for distributed baluns. In particular, in a distributed balun of the type to which the present invention applies, the respecting of the winding lengths according to one quarter of the wavelength is necessary for to the proper operation of the balun. If this may be convenient for a unity impedance ratio, a non-unity impedance ratio generally requires inductive elements of values different from one another. It has been envisaged to increase the number of spirals on one side with respect to the other of the transformer (of one branch with respect to the other), which causes an increase in the inductances on one side with respect to the other, and thus a change in the impedance ratio. Such a solution is however incompatible with the proper operation of a distributed balun since the number of spirals must preferentially be the same on the common mode side and on the differential mode side to comply with the other balun characteristics.